In recent years, devices have come into widespread use which subject an image to compression encoding by employing an encoding system handling image information as digital signals, and at this time compress the image by orthogonal transform such as discrete cosine transform or the like and motion compensation, taking advantage of redundancy which is a feature of the image information, in order to perform highly efficient transmission and accumulation of information. Examples of this encoding method include MPEG (Moving Picture Expert Group) and so forth.
In particular, MPEG2 (ISO/IEC 13818-2) is defined as a general-purpose image encoding system, and is a standard encompassing both of interlaced scanning images and sequential-scanning images, and standard resolution images and high definition images. For example, MPEG2 has widely been employed now by broad range of applications for professional usage and for consumer usage. By employing the MPEG2 compression system, a code amount (bit rate) of 4 through 8 Mbps is allocated in the event of an interlaced scanning image of standard resolution having 720×480 pixels, for example. By employing the MPEG2 compression system, a code amount (bit rate) of 18 through 22 Mbps is allocated in the event of an interlaced scanning image of high resolution having 1920×1088 pixels, for example. Thus, a high compression rate and excellent image quality can be realized.
MPEG2 has principally been aimed at high image quality encoding adapted to broadcasting usage, but does not handle lower code amount (bit rate) than the code amount of MPEG1, i.e., an encoding system having a higher compression rate. It is expected that demand for such an encoding system will increase from now on due to the spread of personal digital assistants, and in response to this, standardization of the MPEG4 encoding system has been performed. With regard to an image encoding system, the specification thereof was confirmed as international standard as ISO/IEC 14496-2 in December in 1998.
Further, in recent years, standardization of a standard called H.26L (ITU-T Q6/16 VCEG) has progressed with image encoding for television conference usage as the object. With H.26L, it has been known that though greater computation amount is requested for encoding and decoding thereof as compared to a conventional encoding system such as MPEG2 or MPEG4, higher encoding efficiency is realized. Also, currently, as part of activity of MPEG4, standardization for taking advantage of a function that is not supported by H.26L with this H.26L taken as base to realize higher encoding efficiency has been performed as Joint Model of Enhanced-Compression Video Coding. As a schedule of standardization, H.264 and MPEG-4 Part10 (Advanced Video Coding, hereafter referred to as H.264/AVC) become an international standard in March, 2003.
Further, as an extension thereof, standardization of FRExt (Fidelity Range Extension) including a coding tool necessary for business use such as RGB, 4:2:2, or 4:4:4, 8×8DCT and quantization matrix stipulated by MPEG-2 has been completed in February, 2005. Thus, H.264/AVC has become a coding system capable of suitably expressing even film noise included in movies, and has been employed for wide ranging applications such as Blue-Ray Disc (registered trademark) and so forth.
However, nowadays, needs for further high-compression encoding have been increased, such as intending to compress an image having around 4000×2000 pixels, which is quadruple of a high-vision image. Alternatively, needs for further high-compression encoding have been increased, such as intending to distribute a high-vision image within an environment with limited transmission capacity like the Internet. Therefore, with the above-mentioned VCEG (=Video Coding Expert Group) under the control of ITU-T, studies relating to improvement of encoding efficiency have continuously been performed.
For example, with the MPEG2 system, motion prediction and compensation processing with ½ pixel precision has been performed by linear interpolation processing. On the other hand, with the H.264/AVC system, prediction and compensation processing with ¼ pixel precision using a 6-tap FIR (Finite Impulse Response Filter) filter has been performed.
As to this prediction and compensation processing with ¼ pixel precision, in recent years, studies for further improving efficiency of the H.264/AVC system have been performed. As one of coding systems for this, with NPL 1, motion prediction with ⅛ pixel precision has been proposed.
Specifically, with NPL 1, interpolation processing with ½ pixel precision is performed by a filter [−3, 12, −39, 158, 158, −39, 12, −3]/256. Also, interpolation processing with ¼ pixel precision is performed by a filter [−3, 12, −37, 229, 71, −21, 6, −1]/256, and interpolation processing with ⅛ pixel precision is performed by linear interpolation.
In this way, motion prediction using interpolation processing with higher pixel precision is performed, whereby prediction precision can be improved, and improvement in encoding efficiency can be realized particularly with a relatively slow motion sequence having high texture in resolution.
Incidentally, as one factor for the H.264/AVC system realizing high encoding efficiency as compared to the MPEG2 system according to the related art and so forth, adoption of a next-described intra prediction system has been proposed.
With the H.264/AVC system, there have been defined intra prediction modes for luminance signals of nine kinds of prediction modes in block units of 4×4 pixels and 8×8 pixels, and four kinds of prediction modes in macro block units of 16×16 pixels. With regard to color difference signals, there have been defined intra prediction modes of four kinds of prediction modes in block units of 8×8 pixels. The intra prediction modes for color difference signals may be set independently from the intra prediction modes for luminance signals. Note that the kinds of the prediction modes correspond to directions indicated with numbers 0, 1, 3 through 8 in FIG. 1. The prediction mode 2 is average value prediction.
Such an intra prediction system has been employed, thereby realizing improvement in prediction precision. However, with the H.264/AVC system, as illustrated in the directions in FIG. 1, intra prediction in increments of 22.5 degrees alone is performed. Accordingly, in the event that the inclination of an edge has an angle other than that, improvement in encoding efficiency is restricted.
Therefore, with NPL 2, proposal has been made for further improvement in encoding efficiency wherein prediction is performed with a finer angle than 22.5 degrees.